I think a couple of vendors were showing off their DDR4 at CES2014.
I would also like to see what it has to offer for IGPUs.
I mean, when's the last time you saw a GDDR4 video card?

GDDR5, which is used on current GPUs, replaced GDDR4. Both of were derived from DDR3 memory. GDDR numbering basically is not related to DDR numbering.

GDDR6 is currently in development. The other form of memory being pursed is 3D stacked DRAM, which is currently on Nvidia's roadmap for Volta. In the mean time it seems wider buses (the largest Maxwell is rumored to also be 512 bit like the 290/290x. "Mid range" cards will move beyond 256 bit) will be used to increase bandwidth as GDDR5 speeds have basically peaked.